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Reversed Field Pinches

Illustration of the two pathways for RFP development: self-organised, quasi-single-helicity (left) and profile control twists or tears (right). The distortions have been exaggerated to emphasise the phenomena.*

Possible plasma pathways

Policy contextThe most highly developed approach to fusion power, the tokamak, confines hot plasmas through the use of large, superconducting magnets that create a powerful magnetic field. However, the coils are applied on the outside of the chamber, creating physics challenges that must be overcome. In the reversed field pinch (RFP) approach, the current in the plasma generates almost all of the magnetic field, “pinching” the plasma in place without the need for coils to create a magnetic field. This results in a higher efficiency between kinetic and magnetic pressure and a greater density of the magnetic field. Continued R&D focussing on alternative approaches to plasma confinement such as RFP is needed in order to contribute to knowledge of fusion science and accelerate the realisation of fusion power.

BackgroundThe main objective of the Implementing Agreement for a Programme of Research and Development on Reversed Field Pinches (RFP IA) is to share instrumentation, joint experiments and common development of theory and models of RFP. Research activities include assessing the potentials of the reactor configuration; determining plasma confinement scaling; providing state-of-the-art facilities for developing active feedback control of magneto-hydrodynamic stability; exploring inaccessible fusion parameter ranges in order to enhance fusion predictive capabilities; and creating techniques to either sustain steady-state plasma or find a pulsed reactor scenario. There are currently three Contracting Parties.

SpotlightA key challenge to advancing the RFP approach is to reduce instabilities that cause distortion of the plasma shape. Under the RFP IA, substantial progress has recently been made on two potential paths to reduce these irregularities.

One option seeks to optimise the plasma’s own tendency to form a simple helical symmetry, called the quasi-single-helicity regime. Based on recent results, an improved understanding of the quasi-single-helicity phase transition is emerging. Good progress has been made in the RFX1 device to achieve high plasma current and density by coating the plasma-facing carbon wall with lithium. These optimal plasma conditions have also been obtained in the MST2 device though with a lower plasma current than the RFX.

The second path to improved RFP involves controlling the current profile in order to bring the plasma as close as possible to pure toroidal symmetry by reducing twists and tears. Record confinement and plasma density have been achieved using this approach. New measurements in the MST device show that confining ion impurities attains the lowest value possible, the so-called ‘classical confinement’.

These results strengthen the validity of RFP research and the relevance for other fusion devices and experiments worldwide. Examples include the new RFP program at the University of Science and Technology of China (USTC), in Hefei, People’s Republic of China.

1. Refers to the Reversed Field Experiment device operated by the Consorzio RFX (Italy).

2. Refers to the Madison Symmetric Torus device at the University of Wisconsin (United States).

* Schema courtesy of David Terranova.

Current projects

Co-ordinated experiments on the following devices:

EXTRAP T2-R (Sweden)

Madison Symmetric Torus (United States)

TPE-RX (Japan)

Reversed Field Experiment (Italy)

For more information: The RFP IA website is udner development.

Participants

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